Resistor



April 26, 1966 c. J. GANCI 3,248,680

RESISTOR Filed Dec. 11 1962 IN VEN "FOR "4a ,4 rro /vEy United States Patent Ofiiice 3,243,639 Patented Apr. 26, 1966 This invention relates to electrical resistors having a resistive element on a supporting ceramic base or core with a protective coating, if desired. The invention is directed particularly to the composition of the materials forming the resistive element and the ceramic base and the coetficients of expansion of these materials.

In the resistors of this type, the insulating base provides a strong rigid support for the resistive element to firmly hold the element in the given fixed relation to prevent shorting of the resistive element or physical damage to it and maintain the turns of a wire wound element in proper heat radiating relation. The base may have a number of different shapes-tubular, elliptical, disc, rectangular, or other suitable form. The resistive element is mounted on an exterior surface of the base for good heat radiation and secured to the base by attachment to electrical terminals mounted on the base and by interlocking with projections on the base or encircling the base. The essential properties of the base are that it is rigid to maintain the proper relation of the parts of the resistive element and that it is tough or strong to withstand the internal strains and physical shock. In view of the high temperatures to which the base is subjected, it should have a high specific heat and a high thermal conductivity to absorb the heat and rapidly conduct the heat to produce a cooling of the base after the removal of the high heats of the manufacturing process or during actual operation of the resistor. The coefiicient of expansion should also be relatively low in order to withstand rapid changes in temperature, thus having a high ability to withstand thermal shock.

The resistive element may be a resistance wire or a resistance ribbon wound around a tubular base or mounted on the side of a disc-shaped or rectangular-shaped base.

Terminals embedded in the base or encircling the base may be provided for receiving the ends of the wire or ribbon for connection to an external circuit. The resistive element may have a resistance from a few tenths of an ohm to more than a million ohms and from low to high wattages.

The resistive element is preferably covered by a coating such as vitreous enamel for forming a hermetic seal, or cement or the like. The coating adheres to the element and to the base to provide a tough, shock resistant covering electrically isolating the resistive element. In these coatings difficulties are encountered in securing the proper interrelation of the base, resistive element and coating to secure a rugged, durable resistor of an efficient size and inexpensive to manufacture. These members are subjected to high temperatures in the manufacturing operation and in subsequent use. The flow of heat in and out of the resistor creates stresses in the members resulting in the cracking of the coating, breaking of the resistive element and weakening of the supporting base so that it is more readily subject to breakage under shock. One of the main reasons for these difliculties is the rigidity of the base and coating and the difference in the coefiicients of expansion of the members. The coating should have a coefiicient of expansion equal to that of the base or lower than that of the base.

An object of the invention is to provide an electrical resistor with a resistive element and a supporting ceramic base having substantially the same coeflicient of expan- Another object of the invention is to provide a ceramic base supporting a resistive element that has a higher wattage dissipation for a given size than other ceramic bases.

Other and further objects and advantages will be apparent from the following description taken in connection with the drawing in which a sectional view of a tubular wire wound resistor is illustrated.

The electrical resistor shown in the drawing comprises a -tubular base 10 with terminals 11 and 12 at opposite ends and a resistance wire 13 wound therearound and connected to the terminals. A protective coating 14 may be provided covering the core, terminals and resistive element. The core is made of a ceramic comprising substantially 99.5 beryllium oxide.

Example of beryllium oxide composition- Percent BeO 99.5

A1 0 MgO Si0 CaO SrO B a0 FeO, Fe O NiO CaO The core may be formed by mixing the foregoing composition and finely grinding the mixture. The ground mixture is then molded or extruded under pressure into a tubular form and fired at a temperature of approximately 3500 F. to 4000 F. to form a rigid, hard, non-porous ceramic core having an average coefiicient of expansion of approximately 9X10 inches per inch per degree centigrade in the range of 25 C. to 800 C. Various com: positions may be used, but for high heat capacity it is preferred that the composition contains 99.5% beryllium oxide or better. The firing of the molded core causes a recrystallization and binding of the crystals to form a high strength body which can withstand impact and rapid changes in temperature without cracking or breaking. The resultant core also has a high thermal conductivity (same as aluminum at room temperature) permitting a reduction in size of the resistor or an increase in wattage rating over present ceramic mounted resistors of the same size. The beryllium oxide should be in the range of 99% to 100% of the ceramic composition forming the core. The core has a high heat conductivity with a high coefiicient of resistance. Thus the core acts as an excellent conductor for removing heat from resistive elements while functioning as an electrical insulator.

The resistive element placed on the core may have any of the conventional compositions. For example, nickelchromium-iron alloys, such as 65% nickel, 15% chromhim and the balance iron; nickel-chromium alloys, such as nickel chromium 20%; or nickel-copper alloys, such as nickel 55%, copper 45%, can be used with the beryllia ceramic cores. However, it is preferred to use a resistive element with beryllia ceramic cores that has a matching linear coefiicient of thermal expansion, for example, a metal alloy comprising 11% chromium, 35% aluminum, 14% vanadium and the balance titanium is preferred. This metal alloy has a coeflicient of expansion 9.5 to 10X 10- inches per inch per degree Centigrade in the temperature range of 25 C. to 1000 C. This coetficient of expansion is substantially the same as that of the beryllium oxide base, and thus the combination of this alloy on the base produces a very desirable resistor that is easier to manufacture and eliminates strains between the resistive element and the ceramic base and the loosening of the resistive element on the base during the formation of a vitreous enamel coating. In addition, the wire is tough and has a high tensile strength which permits it to be used in a fine form without breaking or deforming.

The illustrated terminals are of the band type and have a metal composition of 46.5% nickel and the balance iron. The coefficient of expansion of the bands with this composition is 8 to 9X10- inches per inch per degree centi-.

grade in the temperature range of 25 C. to 800 C. Of course, other forms of terminals may be used, such as cup-shaped terminals for axial type resistors or pin type terminals embedded in the core.

The resistor or resistors may have various types of protective coating, such as vitreous enamel, cements and the like. The vitreous enamel coatings are preferred since they hermetically seal the terminals and the resistive element and have excellent heat conductivity. Various enamels and compositions may be used depending on various characteristics desired. The coating should have a coeffiecient of expansion less than the coefiicient of expansion of the ceramic base so as to prevent crazing of the coating. An example of a composition having the desired coeflicient of expansion is set forth as follows:

Percent BaO 1 3.29

SiO 46.70 MgO a- .66 ZrO 1.98 B 0 11.16 CaO 1.65

CaF 3.29 Li O 1.98 A1 0 1 7.68 2110 11.50 Cr O 1.65 NiO .66

Ti0 1.65 PbO 6.25

This mixture is fritted and milled to proper mesh and formed into a slurry. The slurry is then applied to the base, wire winding and the terminals of the resistor by dipping the resistor in the slurry, by spraying or by other means. The resistor is then fired with a heating cycle of approximately to minutes at 1650 F. to 1750 F.

Such a coating has a coefficient of expansion of 6X10- inches per inch per degree centigrade. Other enamels with slightly higher expansions can be used, provided it is lower than that of the core.

As previously mentioned, other coatings may be used depending upon the characteristics desired and cost of manufacture. Cements may be used, made of a ceramic grog with sodium or potassium silicates used as bonding agents. Such coatings are tough andinsoluble. By treating the coating with silicone oils and/ or silicone resins, depending on the surface required, a waterproof coating may be formed. In other types of coatings, silicates and Teflon may be mixed with the grogs and pigments as bonding agents. The grogs and pigments may comprise particle size-blended mixtures of clay, mica, color, alumina, beryllia, zirconia and the like. The silicones and Teflon also act as moisture proofing agents. Examples are as follows:

Silicone coating Dow Corning 805 resin 50 Mica (washed) 325 mesh 30 Chrome oxide green 5 Kaolinwater washed-air floated 15 Apply nd .air .dry and bake to 600 F. for 2 hours.

Mix thoroughly and apply. Air dry and bake to 600 F. for three hours.

Teflon Teflon suspensoid, c.c 40 Alcoa tabular alumina T-60, gr. 100 Alcoa A-14 4-00 mesh, gr; 30

Mix thoroughly. Apply and air dry. Sinter at 750 F. for 3 minutes, approximately.

For heat dissipation qualities, the vitreous enamel coating is preferred. However, if desired, resistors may be made without coating. Since the resistive element and the ceramic base have substantially the same coefiicient of expansion, these resistors may be subjected to high heats without the resistive element loosening on the base.

In addition to the titanium alloy resistive element previously described, other titanium metal alloys containing vanadium and aluminum may be used, such as an alloy containing 4% vanadium, 6% aluminum and titanium; these alloys having expansion coefficients in the order p of 9.5 to 10X l0- inches per inch per degree centigrade in the temperature range of 25 C. to 800 C. These alloys also have a specific resistance of about 900 ohms per circular mil foot and a temperature coeflicient of resistivity in the vicinity of parts per million.

In view of the high heat conductivity of the beryllia base, which is in the range of .4 to .6 calorie per degree centigrade per second per centimeter at normal temperatures, the electrical resistors have much higher wattage ratings than similar size resistors formed from other ceramic materials. The beryllia base has a conductivity comparable to aluminum and other metals and yet it functions as a good electrical insulator.

As previously mentioned, the standard resistive wires may be used to form resistive elements, but to provide superior resistors with greater ability to withstand shock and a greater reliability in manufacture since the turns of the wires remain in place, titanium alloy resistive elements may be used. The beryllia base and the titanium alloy resistive element have substantially the same coeflicient of expansion. Thus during the firing of the resistor in forming the protective coating, the turns of the resistive element remain in place and do not produce shorts. In describing the resistive element, it is understood that a wire element refers to either a circular wire or a ribbon type wire.

I claim:

1. An electrical resistor comprising a rigid beryllia base composed of 99 to 100% beryllium oxide and having a coefficient of expansion of approximately 9X10 inches per inch per degree centigrade in the temperature range of 20 to 800 degrees centigrade, terminal means on said base and a wound resistive element on said base and engaging said terminal means, said element comprising a titanium alloy containing aluminum and vanadium having a coefficient of expansion substantially the same as said base.

2. An electrical resistor as set forth in claim 1 wherein said titanium alloy comprises 11% chromium, 3.5% aluminum, 14% vanadium and 71.5% titanium.

3. An electrical resistor as set forth in claim 1 wherein said titanium alloy comprises 4% vanadium, 6% aluminum and 90% titanium.

4. An electrical resistor comprising a rigid sintered substantially pure beryllia base with a coeflicient of expansion of approximately 9 X 10- inches per inch per degree centigrade in the temperature range of20 to 800 degrees centigrade, a wound resistive element on said base and having a coeflicient of expansion substantially equal to said base,

5 spaced terminals on said base connected to said resistive element for passing current through said resistive element and having a coeflicient of expansion substantially equal to the coefficient of expansion of said base to provide the resistor with a high wattage capacity and operability at high temperatures.

5. An electrical resistor comprising a rigid sintered substantially pure beryllia base with a coefficient of expansion of approximately 9X10 inches per inch per degree centigrade in the temperature range of 20 to 800 degrees centigrade, a Wound resistive element on said base having a coefficient of expansion substantially equal to said base, spaced terminals on said base connected to said resistive element for passing current through said resistive element and having a coefficient of expansion substantially equal to the coefficient of expansion of said base and a vitreous enamel coating firmly adhering to said base and having a coeflicient of expansion less than the coefiicient of expansion of said base to provide the resistor with a high wattage capacity and operability at high temperatures.

References Cited by the Examiner UNITED STATES PATENTS 1/1934 Young 338264 X 11/1934 Ruben 338257 X 4/ 1937 Von Wedel.

5/1943 Stupakoff et al. 174-152.4

8/1947 Deyrup 338352 10/1954 Ungewiss 338302 2/1956 Lindenblad 136-42 X 8/1957 Kessler et a1 75--175.5 X 12/1958 Busch et al 7S175.5 X

8/ 1959 Patrichi.

9/1959 Abkowitz 75175.5 X

6/1960 Jaffee 75175.5 X

7/1960 Schurecht 10646 9/1964 Vordahl 75-1755 FOREIGN PATENTS 9/ 1937 Great Britain.

8/ 1946 Great Britain.

2/ 1957 Great Britain.

RICHARD M. WOOD, Primary Examiner. 

1. AN ELECTRICAL RESISTOR COMPRISING A RIGID BERYLLIA BASE COMPOSED OF 99 TO 100% BERYLLIUM OXIDE AND HAVING A COEFFICIENT OF EXPANSION OF APPROXIMATELY 9X10**-6 INCHES PER INCH PER DEGREE CENTIGRATE IN THE TEMPERATURE RANGE OF 20 TO 800 DEGREES CENTIGRADE, TERMINAL MEANS ON SAID BASE AND A WOUND RESISTIVE ELEMENT ON SAID BASE AND ENGAGING SAID TERMINAL MEANS, SAID ELEMENT COMPRISING A TITANIUM ALLOY CONTAINING ALUMINUM AND VANADIUM HAVING A COEFFICIENT OF EXPANSION SUBSTANTIALLY THE SAME AS SAID BASE. 